This invention relates generally to an electromagnetic clutch and, more particularly, to an improved clutch rotor and armature plate which increases the torque transfer of the clutch.
Electromagnetic clutches are well known in the art and are often used for controlling the transfer of power from an automobile engine to the refrigerant compressor of the automobile's air conditioning system. The basic construction and operation of these types of clutches are also well known in the prior art. Examples of such clutches may be found in prior U.S. Pat. Nos. 3,044,594 and 3,082,933.
With reference to FIGS. 1 and 2, an electromagnetic clutch of the type known in the prior art will be described. FIG. 1 is a cross-sectional view showing an electromagnetic clutch mounted on a refrigerant compressor. The clutch is disposed on the outer peripheral portion of tubular extension 2 which projects from an end surface of compressor housing 1. Tubular extension 2 surrounds drive shaft 3 of the compressor. Drive shaft 3 is rotatably supported in compressor housing 1 by bearings (not shown). The clutch assembly includes rotor 5 which is rotatably mounted on tubular extension 2 through bearing 4. Rotor 5 may be driven by an automobile engine (not shown) via belt 6. Rotor 5 is also provided with a plurality of concentric arcuate magnetic pole faces 5a at its axial end. The outer terminal end of drive shaft 3 extends from tubular extension 2 to hub 7 which is fixed on the end of drive shaft 3. Armature plate 8 is provided with concentric arcuate magnetic pole faces 8a and is flexibly joined to hub 7 by a plurality of leaf springs 9 in such a fashion that pole faces 8a are positioned facing pole faces 5a of rotor 5 with a predetermined axial air gap between them. Electromagnet 10 is mounted on compressor housing 1 eccentrically with drive shaft 3. Electromagnet 10 is fixed within annular hollow portion 5b which is formed in rotor 5 and has a surrounding air gap to facilitate passage of the magnetic field generated when electromagnet 10 is activated for attracting pole faces 8a of armature plate 8 to pole faces 5a of rotor 5. Thus, when electromagnetic coil 101 of electromagnet 10 is energized, pole faces 8a are attracted to pole faces 5a. Drive shaft 3 is then rotated when rotor 5 is turned by the engine. When electromagnet coil 101 of electromagnet 10 is not energized, pole faces 8a of armature plate 8 are separated from pole faces 5a of rotor 5 due to the spring bias of leaf springs 9. Thus, rotor 5 is permitted to rotate in response to the engine output, but drive shaft 3 is not driven.
In electromagnetic clutches of the type shown in FIG. 1, pole faces 5a of rotor 5 and faces 8a of armature plate 8 include a frictional surface for transmitting torque from rotor 5 to armature plate 8 when the pole face surfaces are engaged with each other. These frictional surfaces are finished by a polishing process. Therefore, each surface has an uneven irregular face as shown in FIG. 2. During the early stages of initial use of the clutch, the frictional surfaces of pole faces 5a and 8a securely engage with one another due to the roughness of each surface. Accordingly, a high level of torque is transferred. However, after the initial stages of use, the frictional surfaces suddenly begin to wear to a smoother finish, making secure engagement difficult. Accordingly, torque is no longer efficiently transferred from rotor 5 to armature plate 8. After subsequent engagements of pole faces 5a and 8a, their frictional surfaces tend to become rough again and the efficiency of torque transfer gradually increases. The decrease in torque transfer during the initial stages of clutch use and the gradual increase in torque transfer after subsequent engagements of the rotor and armature plate pole faces is shown in graph form in FIGS. 7(a) and 7(b). Graph I in both figures illustrate this phoenomenon in electromagnetic clutches known in the prior art and graphs II and III illustrate this phoenomenon in the electromagnetic clutch of the present invention as will be explained below.
In order to overcome the above-described disadvantage with respect to the loss of torque transfer efficiency during the initial stages of clutch operation, particularly the sudden reduction in torque transfer, the size of the frictional surfaces in electromagnetic clutches known in the prior art have been increased to provide more engagement surface for the rotor and armature plate pole faces. In other prior art electromagnetic clutches, the size of the magnetic coil is enlarged to increase the strength of its magnetic field. Accordingly, the rotor and armature plate pole faces are forced into more secure engagement. Although increasing the size of the frictional surface and/or the size of the magnetic coil improves the efficiency of torque transfer, these measures create other problens. For example, the size and weight of the clutch is significantly increased. In addition, the cost of the clutch is increased as well. These additional problems are alleviated somewhat by attempting to limit the increase in the size of the frictional surfaces and magnetic coil by frabricating the frictional surfaces with metal having a high coefficient of friction. However, such metals are very expensive and, therefore, are not a satisfactory alternative.